Friction clutches as used heretofore, usually have opposing surfaces on the driving and driven members which are moved into engagement with one another to provide a friction drive for connecting the members. While clutches of this kind have good control features they require a substantial force to prevent the friction members from slipping relative to one another when delivering relatively high torques. When an electromagnet is used for moving the members into engagement with one another a relatively high current is required.
To avoid this problem, overrunning clutches were used for delivering high torques and usually included rollers or balls between the driving and driven members. One of the members had cam surfaces for engaging the rollers or balls to provide a driving connection between the members. As the rollers or balls rolled onto the cam surface they wedged between the members and attempted to ride higher on the cam surfaces thereby locking the members together. In some instances, especially after the surfaces became worn, it was difficult to disengage the clutch.